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SODIUM SULPHATE                                  105

to 100 c.c. ; 10 c.c. of the solution ( = i gram of substance) are titrated with
N-alkali in presence of methyl orange : I c.c. N-alkali = 0-031 gram Na20.

If the total Na20, thus obtained, is diminished by the quantity corre-
sponding with the1 stannic oxide found (i part of SnO2 requires 0-4106
part of Na20), the free alkali is obtained.

7. Determination of the Tin.I gram is dissolved in water and hydro-
chloric acid, the solution being then allowed to react with a few pure alumi-
nium turnings for about half an hour in the cold. The liquid is then heated
with a further quantity of cone, hydrochloric acid in a current of carbon
dioxide until the spongy tin which has separated completely redissolves ;
the subsequent procedure is as indicated under " Stannous Chloride"
(quantitative determination).


* *

Commercial sodium stannate is never completely soluble in water, but it is
required to dissolve to as great an extent as possible. The percentage of tin
may vary from 30 to 44 (theoretical, 44-85), equal to 38-56% SnO2. The con-
tent in free alkali may be variable (up to 5%), as also may the proportions of
chloride, sulphate, arsenate and tungstate (commercial stannates have been
met with containing 2-50% of sodium, chloride and 15-20% of arsenate).

For dyeing purposes, absence of iron and little free alkali are particularly


Na2S04 + ioH20 = 322 ;   Na2S04  142

The pure salt forms colourless crystals (+ ioH20), soluble in about 3
parts of cold water. The crude salt for technical purposes is also sold and
forms white or yellowish anhydrous powder or fused masses.^ The more
common impurities are : sodium chloride and bisulphate, magnesium, cal-
cium and ammonium salts, arsenic, heavy metals and insoluble substances.
For the analysis of the crystallised salt it is usually sufficient to test for
the above impurities by the methods indicated under " Potassium Sul-
phate " (the arsenic test is made on i gram dissolved in 3 c.c. of water,
which should not give a brown coloration with 5 c.c. of Bettendorf's reagent
within an hour). With the crude salt, the following determinations are
made :

1.  Moisture.2-3 grams are gently ignited and reweighed.

2.  Free Acid (bisulphate).20 grams are dissolved in water to 250 c.c.,
50 c.c. of the solution (=4 grams of substance) being titrated with N-alkali
in presence of methyl orange :  i c.c. N-alkali corresponds with i% S03.

3.  Sodium Chloride.50 c.c. of the solution (= 4 grains of substance)
are neutralised exactly with N-alkali (the quantity necessary is known from
test 2), a little potassium chromate added and the liquid titrated with N/io-
silver nitrate.   Each c.c. of N/io-AgN03 corresponds with 0-146 % NaCl.

4.  Iron.10 grams are dissolved in water and the solution treated with
sulphuric acid and pure zinc, the iron thus reduced being titrated with per-
manganate in the ordinary way.    If the iron is present in very small pro-
portion, the colorirnetric method used with aluminium sulphate may be
employed.5% Cl.RIDE.   With ammonia  soda, 20 c.c. (=2 grams of